The present invention relates to a photopolymerizable material which can be used for image-forming materials such as three-dimensional formation, holography, lithographic printing plate material, color proof, photoresist and color filter, or for photocurable resin materials such, as ink, coating material and adhesive. More specifically, the present invention relates, to a photopolymerizable composition suitable as a lithographic printing plate material capable of so-called direct plate-making, which can directly making a plate from digital signals of a computer or the like using various lasers.
At present, high-output and compact lasers are easily available, which is a solid laser, a semiconductor laser or a gas laser each radiating an ultraviolet ray, a visible ray or an infrared ray at a wavelength of from 300 to 1,200 nm. These lasers are very useful as a light source in the recording for directly making a plate from digital lasers of a computer or the like.
A large number of studies have been made on the recording material having sensitivity to various laser rays and representative examples of the recording material include the followings.
The first is a material which has photosensitive wavelength of 760 nm or more and can be recorded by an infrared laser, and for this material, a positive-working recording material described in U.S. Pat. No. 4,708,925 and an acid catalyst crosslinking-type negative working recording material described in JP-A-8-276558 are known.
The second is a recording material having sensitivity to an ultraviolet or visible laser ray at a wavelength of from 300 to 700 nm and for this material, a large number of recording materials are known, such as a radical polymerization-type negative recording material described in U.S. Pat. No. 2,850,445 and JP-B-44-20189.
On the other hand, materials having sensitivity to light at a short wavelength of 300 nm or less or an electron beam are important particularly as a photoresist material. In recent years, integrated circuits are more and more intensified in the integration and in producing a semiconductor substrate for ultra LSI and the like, working of an ultrafine pattern comprising lines in the width of a half micron or less is involved. To satisfy this, the exposure apparatus for the photolithography is required to emit light of shorter wavelengths and use of a far ultraviolet ray or an excimer laser ray (e.g., XeCl, KrF, ArF) is being studied. Use of even an electron beam is being attempted for the formation of an ultrafine pattern. The electron beam in particular is highly promising as a light source for the pattern-formation technologies in the coming generation.
These image-forming materials all have a common problem to be solved and this is to what extent the ON-OFF of the image can be expanded in the energy-irradiated area and the non-irradiated area, namely, to simultaneously attain both high sensitivity and storage stability of the image-forming material.
Usually, the radical photopolymerization system has high sensitivity but due to the polymerization inhibition by oxygen in air, the sensitivity greatly decreases. Therefore, means of providing a layer capable of shutting out oxygen on the image-forming layer is employed. However, when a layer capable of shutting out oxygen is provided, fogging in turn occurs due to the dark polymerization and the storage stability is deteriorated. Accordingly, it is heretofore difficult to simultaneously attain both high sensitivity and storage stability, and none of conventional techniques is satisfied in the results. Thus, a new technique not known in the past is being demanded.
Accordingly, an object of the present invention is to provide a photopolymerizable composition capable of successfully ensuring both high sensitivity and excellent storage stability, as a radical photopolymerization-system composition which is highest in the sensitivity and very promising out of the image-forming techniques. Particularly, the object of the present invention is to provide a photopolymerizable composition suitable as a lithographic printing plate material capable of making a printing plate directly from digital data of a computer or the like by the recording using a solid laser or semiconductor laser which emits an ultraviolet ray, a visible ray or an infrared ray.
As a result of extensive investigations, the present inventors have found that when a compound having a specific polymerizable group is used in the photopolymerizable composition, the above-described object can be attained. More specifically, the present invention has the following constructions.
A photopolymerizable composition comprising a compound having a structure represented by the following formula (I) and a photopolymerization initiator: 
wherein X1 and X2 each independently represents a hetero atom or a halogen atom, Ra and Rb each independently represents a hydrogen atom, a halogen atom, a cyano group or an organic residue, and X1 and X2, Ra and Rb, or X1 and Ra or Rb may combine with each other to form a cyclic structure.
Conventionally, the most general radical polymerizable compound is a monomer, oligomer or polymer having an acrylic acid ester group, an acrylic acid amide group, a methacrylic acid ester group or a methacrylic acid amide group each having high polymerizability. However, these are subject to the polymerization inhibition due to oxygen and when such a compound is used in a photopolymerizable composition, the object of simultaneously attaining both high sensitivity and storage stability cannot be attained.
With respect to the constituent component of the polymer, compounds (monomer) having an xcex1-hetero-substituted methyl acryl group or an xcex1-halogen-substituted methyl acryl group are known, each group having polymerizability comparable to the acrylic type. Unlike an itaconic acid group or xcex1-alkyl acryl group having a substituent at the same xcex1-position but having low polymetizability, those compounds are considered to be improved in the polymerizability as a constituent component of a polymer due to the electronic effect and the steric effect of a hetero atom or halogen atom substituted to the xcex1-position.
The present inventors have-found that when a compound containing a structure having a polymerizable group substituted at the xcex1-position by a hetero atom or a halogen atom is used together with a photopolymerization initiator in a photopolymerizable composition, the problem of conventional radial polymerizable compounds, namely, the above-described polymerization inhibition by oxygen can be greatly reduced and both high sensitivity and excellent storage stability which are commonly demanded for photopolymerizable compositions can be simultaneously attained.
The mechanism for reducing the polymerization inhibition by oxygen is not clearly known, however, the compound having a structure represented by formula (I) for use in the present invention is extremely small in the stop rate constant, though the polymerization growth rate constant is not so high as compared with conventional acryl or methacryl types, therefore, a reaction with oxygen difficultly occurs at the time of chain propagation, as a result, the compound is considered to become unsusceptible to the effect of polymerization inhibition by oxygen.